Various tools have rotating cutters which modify the shape of a work piece. An example of such a tool is a tapper which cuts the bore of a work piece to provide internal threads. Another example is a milling machine, in which a rotating cutter removes metal from the surface of a work piece to provide a desired shape.
In such tools, the cutter material is harder than the work piece in order to effect cutting of the latter. As a general rule, the harder the cutter material is, the more brittle is the cutter, and the cutter becomes prone to fracture. The problem can be alleviated by hardening only the outer surface of the cutter leaving a core which is softer but tougher. However, it is difficult to harden only the cutting surface in small diameter taps. Moreover, the cutter is prone to fracture if too much work piece material is sought to be removed in a single pass, no matter how hard or soft the work piece material is.
Taps employed to cut internal threads in a work piece are particularly vulnerable to breakage, for a variety of reasons. If, for example, the bore in the work piece is slightly undersized, torsional stress is increased, frequently resulting in a broken tap. The machine must be shut down, the broken tap removed, and a new one inserted before operations can resume. Oftentimes, multiple tapping heads are used at a single station in high speed production lines, increasing the frequency of tap breakage.
Self-tapping screws for insertion in metallic or non-metallic work pieces, such as concrete, have much the same properties as taps for cutting threads in metallic work pieces. They are hard and brittle, and easily broken.